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U-M BIOLOGY 172 - Nucleic Acids and DNA Replication

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DNA can transfer biological characteristics from one organism to another.Question: What is the nature of the genetic material?BIO 172January 16, 2019Nucleic Acids and DNA ReplicationReadings 3.1-3.3 and 12.1-12.21. Deoxyribonucleic acid (DNA) stores and transmits genetic information.2. DNA is a polymer of nucleotides and forms a double helix.3. Transcription is the process by which RNA is synthesized from a DNA template.4. In DNA replication, a single parental molecule of DNA produces two daughter molecules.5. The replication of linear chromosomal DNA requires mechanisms that ensure efficient and complete replication. Deoxyribonucleic acid (DNA): A linear polymer of four subunits; the information archive in all organisms. Double helix: The structure formed by two strands of complementary nucleotides that coil around each other. First, DNA stores information. Genetic information: Information carried in DNA, organized in the form of genes. Gene: The unit of heredity; the stretch of DNA that affects one or more traits in an organism, usually through an encoded protein or noncoding RNA. Gene expression: The production of a functional gene product. Gene regulation: The various ways in which cells control gene expression. Second, DNA transmits genetic information from one generation to the next. 3.1 Major Biological Functions of DNA DNA can transfer biological characteristics from one organism to another. Question: What is the nature of the genetic material? Background: In the 1920s, it was not clear what biological molecule carries genetic information. Fred Neufeld, a German microbiologist, identified several strains of the bacterium Streptococcus pneumoniae, one of which was virulent and caused death when injected into mice, and another which was nonvirulent and did not cause illness when injected into mice. Experiment: He noted that nonvirulent bacteria do not cause mice to get sick and virulent bacteria that had been killed do not cause mice to get sick, but when the two were mixed, the injected mice got sick and died. Conclusion: One strain of bacteria (nonvirulent) can be transformed into another (virulent) by an unknown molecule from the virulent cells. In other words, the unknown molecule carries information that causes virulence. Transformation: The conversion of cells from one state to another, as from nonvirulent to virulent, when DNA released to the environment by cell breakdown is taken up by recipient cells. In recombinant DNA technology, the introduction of recombinant DNA into a recipient cell. Background: Oswald Avery, Colin MacLeod, and Maclyn McCarty also studied virulence in pneumococcal bacteria. They recognized the significance of Griffith’s experiments and wanted to identify the molecule responsible for transforming nonvirulent bacteria into virulent ones. Conclusion: DNA is the molecule responsible for transforming nonvirulent bacteria into virulent bacteria. This experiment provided a key piece of evidence that DNA is the genetic material. DNA molecules are copied in the process of replication. The process of copying DNA so genetic information can be passed from cell to cell orfrom an organism to its progeny. An unrepaired error in DNA replication results in a mutation, which is a change in thegenetic information in DNA. - While most mutations in genes are harmful, rare favorable mutations are essential in the process of evolution because they allow populations of organisms to change through time and adapt to their environment. Genetic information flows from DNA to RNA to protein. In specifying the amino acid sequence of proteins, DNA acts through an intermediary molecule known as ribonucleic acid (RNA), another type of linear polymer.  The flow of information from DNA to RNA to protein has come to be known as the central dogma of molecular biology.- Through the years, some exceptions to this “dogma” have been discovered, including the transfer of genetic information from RNA to DNA (as in HIV, whichcauses AIDS), from RNA to RNA (as in replication of the genetic material of influenza virus), and even from protein to protein (in the unusual case of disease-causing molecules called prions).  The first step in this process is transcription, in which the genetic information in a molecule of DNA is used as a template, or pattern, to generate a molecule of RNA.  The term “transcription” is used because it emphasizes that both molecules use the same language of nucleic acids. - Transcription is the first step in gene expression, which is the production of a functional gene product. - The second step in the readout of genetic information is translation, in which a molecule of RNA is used as a code for the sequence of amino acids in a protein. The term “translation” is used to indicate a change of languages, from nucleotides that make up nucleic acids to amino acids that make up proteins. The processes of transcription and translation are regulated, meaning that they do not occur at all times in all cells, even though all cells in an individual contain the same DNA.  Genes are expressed, or “turned on,” only at certain times and places, and not expressed, or “turned off,” at other times and places. - In multicellular organisms, for instance, cells are specialized for certain functions, and these different functions depend on which genes are on and whichgenes are off in specific cells.  Muscle cells express genes that encode for proteins involved in muscle contraction, but these genes are not expressed in skin cells or liver cells, for example. - In prokaryotes, transcription and translation occur in the cytoplasm, but in eukaryotes, the two processes are separated from each other, with transcription occurring in the nucleus and translation in the cytoplasm.  The separation of transcription and translation in time and space in eukaryotic cells allows for additional levels of gene regulation that are not possible in prokaryotic cells. 3.2 Chemical Composition and Structure of DNA A DNA strand consists of subunits called nucleotides. The elegant shape of the twisting strands relies on the structure of DNA’s subunits, called nucleotides. - Nucleotides consist of three components: a 5-carbon sugar, a base, and one or more phosphate groups. Each component plays an important role in DNA structure.  The 5-carbon sugars and phosphate groups form the


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